1. Technical Field
This invention relates to directional microphones and more particularly, to an improved apparatus and method for calibrating these microphones.
2. Description of the Prior Art
Gradient microphones have been used for some time to selectively accept sound energy from a desired direction while excluding or attenuating sound energy from other directions. Recently, a class of gradient microphones has been developed which utilize two gradient (differential) microphones located along an axis to further enhance the directionality effects. Such microphone systems frequently employ an electronic delay at the output of one of two first-order gradient microphones whose output is then combined with the output of the second. One arrangement of such a system is described by G. M. Sessler, J. E. West and R. A. Kubli, Unidirectional, Second-Order Gradient Microphone, J. Acoust. Soc. Am., Volume 86, No. 6,2063-2067 (December 1989).
The attractiveness of second-order microphone systems is currently diminished by their relatively complicated mechanical and electrical design requirements. These microphone systems have a tendency toward performance degradation if the sensitivity of the two first-order microphones which comprise the compound second-order microphone system differs by more than a small amount. The degradation appears as a reduction of the microphone's directionality, accompanied by the appearance of undesirable side and rear lobes. There is a need, therefore, to obtain closely-matched first-order microphones and/or to carefully calibrate the associated microphone preamplifiers to achieve near-exact gain balance. Both of these approaches are costly. To obtain microphones matched to the degree of accuracy required (better than 0.5 dB), large numbers of microphones must be accurately calibrated and sorted into "bins", whose contents may then be sold as matched. Since this process involves the testing of many microphones to high accuracy, the use of matched microphones results in high system cost.
Alternatively, microphones less well-matched may be used with microphone preamplifiers whose gain can be accurately adjusted to compensate for the microphone mismatch. This process requires specialized equipment, an anechoic environment, and hand labor to perform the adjustment. The procedure involves use of a point sound source located in the far-field, in line with the axis of the microphone. With the rear null of the microphone pointed toward the source, the gain of the preamplifier connected to one of the first-order microphones is adjusted to minimize the output of the composite microphone.
Both of these alternatives still have limitations: As the microphones and amplifiers age, their sensitivity and gain may change unpredictably. Effects such as mechanical shock, temperature, humidity, and operating voltage may also contribute to loss of matching accuracy. This opens the possibility that even with exact factory calibration, microphone quality may degrade over time.